In recent years, there have been known wireless power supply systems that supply electric power using electromagnetic induction, magnetic field resonance or the like from a power transmitter including a power-transmitting coil to a power receiver including a power-receiving coil. In such systems, magnetic flux generated by the power-transmitting coil is interlinked with the power-receiving coil, whereby electric power is transmitted between the coils. For this reason, the efficiency of power transmission (transmission efficiency) is influenced by a positional relationship between the power-transmitting coil and the power-receiving coil.
The battery charging of an electric vehicle has attracted attention as one application destination of the wireless power supply system, and the power receiver is mounted on the vehicle in this case. However, there is a limitation in driving accuracy (stop accuracy), and it is difficult to park a vehicle in accurate alignment with a predetermined position for the purpose of charging. For this reason, there is the possibility of a positional relationship between the power-transmitting coil and the power-receiving coil varying each time a vehicle is parked. The relative positions of the power-transmitting coil and the power-receiving coil deviate from a desired positional alignment, and thus the efficiency of transmission may decrease due to a change in a coupling coefficient between the coils.
Hitherto, a technique for limiting a decrease in the efficiency of charging when a positional misalignment between coils occurs has been proposed (see, for example, Patent Document 1). A power supply device (power transmitter) disclosed in Patent Document 1 changes the frequency of AC power which is supplied from an inverter circuit (power source) to a power-transmitting coil, when the efficiency of transmission decreases due to the occurrence of positional misalignment. The power supply device achieves an improvement in the efficiency of charging through this frequency change.